1. Field of the Invention
The invention relates to devices for control of powder supply in a crystal growing apparatus.
2. The Prior Art
Several devices for supplying powder to an apparatus for crystal growing according to the Verneuil process are known as, for example, from U.S. Pat. No. 3,917,459, as well as German Patent application P 25 24 783. Generally, Verneuil crystals are produced by supplying a powder to a crystalline material in a flame which melts the powder and directs the melt to a seed crystal being impinged upon by the flame. During the process, the seed crystal is melted at its top or upper surface by the impinging flame and is gradually withdrawn from the region of the flame while, simultaneously, a supply of crystalline material is fed to the melt so that the material progressively crystallizes out of the melt onto the seed crystal. The newly formed, predominantly mono-crystalline material forms the desired crystal as an elongation of the seed crystal.
In known manner of powder conveyance from a supply container, very fine powder to be used in the crystal growth passes by gravity through a bottom screen of the supply container. A second screen having a very fine mesh passes only very fine-particled powder to a flame thereunder. The screens are oscillated to facilitate the passing of powder therethrough. The oscillations, especially vertical oscillations, are known to make the powder feed from the supply container into the flame non-uniform. Detailed studies have shown that vertical oscillations in the air column in the space between the second screen and the supply container cause uncontrolled powder discharges from the supply container.